Optical grating coupler

ABSTRACT

An optical grating is disposed on a waveguide to redirect light from the interior of the waveguide through the opposite side of the waveguide from the grating. In one embodiment the waveguide, the grating, and an optical sensor are combined in a single monolithic structure. In another embodiment, an absorbing layer is directly connected to the waveguide in the region of the grating. In still another embodiment, efficiency of the grating is improved by having a high index contrast between the refractive index of the grating and the refractive index of the cladding disposed over the grating, and by having an appropriately sized discontinuity in the grating.

BACKGROUND

[0001] 1. Technical Field

[0002] An embodiment of the invention relates generally to optics, andin particular relates to optical grating couplers.

[0003] 2. Description of the Related Art

[0004] Optical gratings are frequently used to redirect light in awaveguide into an optical detector external to the waveguide. Light thathas been traveling transversely through the waveguide by reflecting offthe waveguide's inner surface at shallow angles may be redirected sothat it strikes the inner surface of the waveguide at a sharper anglethat is greater than the critical angle of incidence, thus allowing thelight to escape through the surface. After escaping, the light mayimpinge upon a detector. The detected light may then be used for variouspurposes, such as to receive an encoded communications signal that wastransmitted through the waveguide. Unfortunately, this process mayexhibit poor efficiency, with a large part of the redirected light notreaching the detector. Further, the cost of manufacturing thedetector/optical coupler may be excessive due to the need to manufactureseveral items separately and then assemble them into a completedassembly.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] The invention may be understood by referring to the followingdescription and accompanying drawings that are used to illustrateembodiments of the invention. In the drawings:

[0006]FIG. 1 shows a cross-section of a grating coupler, according toone embodiment of the invention.

[0007]FIG. 2 shows a cross-section of a grating coupler, according toone embodiment of the invention.

[0008]FIGS. 3A-3F show the fabrication of the grating coupler of FIG. 2,according to one embodiment of the invention.

[0009]FIG. 4 shows a cross section of an integrated photodetector,according to one embodiment of the invention.

[0010]FIGS. 5A, 5B show a grating coupler with a discontinuity in thegrating, according to one embodiment of the invention.

[0011]FIG. 6 shows a graph of optical coupling using a grating couplerwith a discontinuity and a high index contrast, according to oneembodiment of the invention.

DETAILED DESCRIPTION

[0012] In the following description, numerous specific details are setforth. However, it is understood that embodiments of the invention maybe practiced without these specific details. In other instances,well-known structures and techniques have not been shown in detail inorder not to obscure an understanding of the description.

[0013] References to “one embodiment”, “an embodiment”, “exampleembodiment”, “various embodiments”, etc., indicate that theembodiment(s) of the invention so described may include a particularfeature, structure, or characteristic, but not every embodimentnecessarily includes the particular feature, structure, orcharacteristic. Further, repeated use of the phrase “in one embodiment”does not necessarily refer to the same embodiment, although it may.

[0014] Some figures show cross sections of various structures. Thefigures are not drawn to scale, and no inference should be drawn as tothe relative dimensions of these structures based on the relativedimensions in the drawings.

[0015] This disclosure uses the following definitions, which may or maynot be used in this manner outside this document:

[0016] Connected—denotes direct physical or electrical contact.

[0017] Coupled—denotes either direct or indirect physical or electricalcontact, with “indirect” indicating that other elements may be betweenthe coupled elements.

[0018] Optical coupler—a structure in which light is redirected from theinterior of a waveguide to a light sensor/detector.

[0019] Waveguide—a solid device used to internally convey light by usinginternal reflections from the surfaces of the waveguide to keep all ormost of the light from escaping, except in designated areas.

[0020] Grating—a structure of parallel ridge-like formations, along aportion of a surface of a waveguide, that redirect a portion of thelight. The ridges may be referred to as ‘teeth’, while the space betweenthe ridges may be referred to as ‘gaps’.

[0021] Cladding—any optical medium, other than the waveguide and thegrating, that is in intimate contact (i.e., direct physical contact)with the surface of the waveguide and/or the grating, and that has adifferent refractive index than the waveguide and/or the grating.Grating-side cladding is the cladding on the side of the waveguide thathas a grating. Receptor cladding is the cladding on the opposite side ofthe waveguide from the grating, so named because a portion of thereceptor cladding may receive some of the light redirected by thegrating. The adjective “receptor” is used herein only to distinguish theindicated cladding from other cladding, and should not be interpreted asa limitation.

[0022] Optical medium—a medium through which light of a desiredwavelength may travel. The waveguide, grating, and cladding referencedabove may all be considered optical media.

[0023] Monolithic structure—a solid structure in which the elements areformed in intimate contact with the adjoining elements, rather thanbeing assembled into a whole after forming the elements.

[0024] One embodiment of the invention has an optical coupler with awaveguide and a light sensor being part of a monolithic structure.Another embodiment integrates a light sensor directly onto the waveguidestructure opposite the grating. Still another embodiment uses a highindex contrast grating with a deliberate discontinuity in the gratingstructure to increase the efficiency of the grating, thus permitting thesize of the grating to be greatly reduced without loss of couplingefficiency.

[0025] Inverted Grating Structure

[0026]FIG. 1 shows a cross-section of a grating coupler, according toone embodiment of the invention. Grating coupler 100 includes awaveguide 120 with a grating 140, a grating-side cladding 130, and areceptor cladding 110. In one embodiment grating coupler 100 redirectslight from the waveguide to a detector. In the embodiment of FIG. 1, thehorizontal arrow depicts light traveling from left to right through thewaveguide 120. In one embodiment waveguide 120 receives the light fromanother transmission medium (e.g., a fiber optic cable), while anotherembodiment may have the waveguide as an integral portion of thetransmission medium. Although some of the light may strike the innersurface of the waveguide 120 (e.g., the surface in contact withgrating-side cladding 130 or the surface in contact with the receptorcladding 110), for most of this light the angle of incidence (the angleat which the light strikes the inner surface) will be less than thecritical angle of incidence (the angle below which light is internallyreflected rather than passing through the surface). The differencebetween the refractive index of the waveguide and the refractive indexof the medium in contact with the waveguide surface determines thiscritical angle. Due to the shallow angle of incidence and the relativerefractive indices of the waveguide 120 and grating-side cladding130/receptor cladding 110, substantially all of the light may reflectback to the interior of the waveguide, thus maintaining high efficiencyin the transmission of light. When the light reaches the area of thegrating, however, the shape of the grating structure may cause a portionof the light to be redirected in substantially different directions.Some of this redirected light will strike the lower surface of thewaveguide (i.e., the surface in contact with receptor cladding 110) at ahigh angle of incidence (i.e., above the critical angle) so that thelight penetrates the surface and goes into receptor cladding 110. Oneembodiment uses air as the receptor cladding 110, but other embodimentsmay use other materials.

[0027] Unlike conventional grating couplers, which have the grating onthe waveguide surface through which light is to be redirected (the“preferred direction” is to the grating side), embodiments of theinvention may have the grating on the opposite side of the waveguidefrom that through which light is to be redirected (the “preferreddirection” is away from the grating side). Also unlike conventionalgrating couplers, the cladding over the grating side has a lowerrefractive index than the cladding on the opposite side of thewaveguide.

[0028] Each of the waveguide 120, grating-side cladding 130, andreceptor cladding 110 have their own refractive index. In oneembodiment, the refractive index n2 of waveguide 120 is higher than therefractive index n1 of receptor cladding 110, which is in turn higherthan the refractive index n3 of grating-side cladding 130. Therelatively high ratio of the refractive index n2 to refractive index n3may cause virtually all light impinging on the n2-n3 interface to bereflected back into the waveguide and/or the grating structure. Thesomewhat lesser ratio of refractive index n2 to refractive index n1 maypermit light striking the n2-n1 interface at a high angle to continueinto receptor cladding 110, while light striking the n2-n1 interface ata relatively shallow angle may be reflected back internally, thuspermitting the waveguide 120 to operate as a substantially losslessconveyor of light in the non-grating area, while effectively redirectingthe light to an external medium in the grating area. The light enteringreceptor cladding 110 in this manner may be handled in various ways(e.g., the light may be captured and detected, the light may continueinto another medium not shown, etc.).

[0029] The structures shown in FIG. 1 may have various dimensions,depending on the specific application. For example, in one embodiment,waveguide 120 may be between approximately 0.2 and approximately 2.0microns (micrometers) in width and thickness, grating 140 may have agrating pitch (the center-to-center spacing of the grating teeth) ofapproximately 0.5 microns, the teeth of the grating may be approximately0.2 microns in height, while the overall grating may be approximately1.5 microns wide and up to a millimeter long along the length of thewaveguide. Other embodiments may use other dimensions.

[0030]FIG. 2 shows a cross-section of a grating coupler 200, accordingto one embodiment of the invention. The embodiment of FIG. 2 comprises awaveguide 220 with a grating 240, a receptor cladding 210, agrating-side cladding 230, an inter-layer dielectric (ILD) 250, a lightsensor 280, an amplifier 270 to amplify the signal produced by thesensor 280, and a substrate 260. Items 210, 220, 230 and 240 may besimilar to, and serve the same basic purposes as, items 110, 120, 130and 140, respectively, in FIG. 1. In FIG. 2, substrate 260 is simplyshown as a layer that provides a base for the remaining structure, butsubstrate 260 may serve other purposes as well, and may contain, orinterface with, other components or layers not shown. In operation, theredirected light that exits waveguide 220 in the area of grating 240 maypenetrate through receptor cladding 210, enter and penetrate through ILD250, and strike light sensor 280. A signal from light sensor 280 mayindicate the intensity of the received light. This signal may beamplified by amplifier 270 and sent to other circuitry where the signalmay be processed in any desirable manner. In one embodiment a sensorcomprises doped silicon in which photons of light create free electronsand holes in the atomic structure, while a voltage placed across thesensor causes a current to flow that is relatively proportional to thequantity of electrons and/or holes. The amplifier may then convert thiscurrent flow into a voltage level sufficient to drive other electroniccircuits. Other embodiments may use other sensors and amplifiers toconvert received light into an electrical signal. Light sensors andsignal amplifiers are well known and are not further described herein.

[0031] A network of conductive traces and interconnecting vias 255,shown in cross-section within ILD 250, may be used to provide electricalpower to amplifier 270 and sensor 280, and to receive signals fromamplifier 270. The traces/vias 255 may provide the conductiveconnections between the amplifier 270 and other circuits not shown, aswell as providing other conductive paths for other purposes. To preventunnecessary loss of light, receptor cladding 210 and ILD 250 may besubstantially transparent to the wavelengths of light used in gratingcoupler 200, and the area of ILD 250 that is above sensor 280 may bekeep clear of traces and vias. Although a single grating, sensor, andamplifier are shown, the structure may include multiples of thesedevices.

[0032] As in FIG. 1, refractive indices n1, n2, and n3 represent therefractive indices of the receptor cladding, the waveguide, and thegrating-side cladding, respectively. And like the embodiment of FIG. 1,the embodiment of FIG. 2 may have n2>n1>n3.

[0033]FIGS. 3A-3F show the fabrication of the grating coupler of FIG. 2,according to one embodiment of the invention. In the illustrated method,the following operations are used, but other methods of fabricatinggrating coupler 200 may also be used.

[0034] In FIG. 3A, a substrate 260 is provided or created. In oneembodiment substrate 260 is a wafer, such as the type of wafer on whichintegrated circuits are formed. In another embodiment, substrate 260 isa layer of material formed directly or indirectly on a wafer. Substrate260 may be comprised of various materials, such as monocrystallinesilicon.

[0035] As further shown in FIG. 3A, a light sensor 280 and an amplifier270 are formed on the substrate 260. When operational, the combinationof light sensor 280 and amplifier 270 may convert light received by thesensor into a voltage delivered by the amplifier, with the amount of thevoltage having a pre-defined relationship to the amount of lightreceived. In the illustrated embodiment the amplifier and sensor areside-by-side and disposed in a recess in the substrate, but otherembodiments may have other configurations (e.g., they may be physicallyseparated, one or both may be fabricated above the surface of thesubstrate, etc.). The formation of light sensors and their associatedsignal amplifiers is well known and is not described further.

[0036] In FIG. 3B, an interlayer dielectric (ILD) 250 is formed abovethe substrate, amplifier, and sensor. In one embodiment, ILD 250 iscomprised of silicon oxide, with embedded traces/vias comprised ofconductive metal, but other embodiments may use other materials. In oneembodiment a volume of the ILD that is directly above the sensor is leftclear of traces and vias to provide a clear light path to the sensor280. For simplicity, only a single electrically conductive path is shownterminating at the amplifier 270, but multiple such conductive paths mayterminate at the amplifier 270 and/or sensor 280. Forming an ILD withconductive elements may involve several successive operations. The ILD250 may have various thicknesses (e.g., less than 10 microns).Techniques for forming ILD's, including multiple levels of conductiveelements, are well known and are not described further.

[0037] In FIG. 3C, receptor cladding 210 is deposited on the ILD. Thematerial of receptor cladding 210 may be chosen for its refractive indexrelative to that of a waveguide created in a subsequent operation. Inone embodiment receptor cladding 210 may comprise silicon oxynitride andmay be between about 1.0 and about 2.0 microns thick, but otherembodiments may use other materials and other thicknesses. The receptorcladding 210 may be deposited through various means (e.g., plasmachemical vapor deposition (PCVD), plasma enhanced chemical vapordeposition (PECVD), low pressure chemical vapor deposition (LPCVD),etc.).

[0038] In FIG. 3D, waveguide material 221 is deposited on receptorcladding 210. In one embodiment waveguide material 221 is comprised ofsilicon nitride, but other embodiments may use other materials. Varioustechniques may be used to deposit the waveguide material 221 (e.g.,PCVD, PECVD, LPCVD, etc.). The thickness of waveguide material 221 mayhave various values, but in embodiments in which the grating is to beetched into the waveguide material 221, the thickness must be greaterthan the height of ridges to be so etched, so that a viable thickness ofwaveguide will still exist beneath the grating.

[0039] In FIG. 3E, a grating 240 is formed. In one embodiment, thegrating 240 is formed by placing photoresist material on waveguidematerial 221, exposing and developing the photoresist to produce apattern of photoresist, etching the portions of waveguide material 221not covered by the pattern, and then removing all remaining photoresistmaterial. In this process, all portions of the surface of the waveguidethat are not to become grating teeth will be etched away to a certaindepth, leaving the waveguide 220 and the raised grating 240 as a singlemonolithic formation. Other embodiments may use other techniques to forma grating (e.g., depositing a material onto the waveguide to form thegrating from the deposited material). Although in some embodiments thewaveguide and the grating are part of the same uniform material, with nostructural or optical boundaries between them, they will continue to bereferred to herein as separate items.

[0040] In FIG. 3F, grating-side cladding 230 may be deposited on thewaveguide 220 and the grating 240. In one embodiment the grating-sidecladding is comprised of silicon oxide, but other embodiments may useother materials. The grating-side cladding may be thick enough to coverall the grating, as well as the waveguide. The grating-side cladding maybe deposited through various means (e.g., PCVD, PECVD, LPCVD, etc.).

[0041] Depending on the application, additional layers of material (notshown or described) may be formed above grating-side cladding 230 and/orbelow substrate 260.

[0042] In the foregoing manner, a complete grating coupler comprisingthe waveguide, the grating, refractive layers above and below thewaveguide, the sensor and sensor electronics, and the interconnectingelectrical paths, may be fabricated into a monolithic unit, using knownor yet-to-be-developed processes common in the fabrication of integratedcircuits. Further, the distance from the waveguide to the sensor may beas small as the combined thicknesses of the receptor cladding 210 andthe ILD 250. In one embodiment, this combined thickness is less thanapproximately 12 microns, but other embodiments may use otherthicknesses. This is in contrast with conventionally assembled opticalcouplers, in which the sensor may be approximately 100 microns from thewaveguide. Since light loss increases with distance from the waveguideto the sensor, the close proximity of the sensor to the waveguide maycause less light loss and thus permit a smaller detector to be used.

[0043] In a particular embodiment, the waveguide is formed of siliconnitride with a refractive index of about 2.0, the grating-side claddingis formed of silicon oxide with a refractive index of about 1.5, and thereceptor cladding is formed of silicon oxynitride with a refractiveindex between about 1.5 and about 2.0—the exact refractive index maydepend on the ratio of oxygen to nitrogen in the silicon oxynitride. Allthree materials may be relatively non-absorbent to the wavelengths oflight to be used in the grating coupler.

[0044] Grating-Enhanced Coupling into Photodetector

[0045]FIG. 4 shows a cross section of an integrated photodetector,according to one embodiment of the invention. In the illustratedembodiment of FIG. 4, photodetector 400 comprises a waveguide 420,grating 440, and absorbing layer 490. Absorbing layer 490 is a layer ofmaterial that absorbs, rather than being transparent to, light energy inthe applicable wavelengths. As with sensor 280 of FIG. 2, in oneembodiment absorbing layer 490 generates free electrons and/or holes inthe atomic structure when light energy is absorbed, and placing apotential across the absorbing layer may cause current to flow in anamount related to the amount of light absorbed. In one embodimentabsorbing layer 490 is comprised of germanium, but other embodiments mayuse other materials (e.g., silicon, silicon germanium, etc.). In oneembodiment absorbing layer 490 is at least 30 microns thick, but otherembodiments may have other thicknesses. In operation, light may travelthrough the waveguide (e.g., from right to left in FIG. 4 as indicatedby the arrow), the light being substantially kept within the waveguidedue to the shallow angle of incidence when light strikes the innersurface, and due to the material of cladding 410, 430, which are inintimate contact with waveguide 420, having a lower refractive indexthat the waveguide. When the internal light reaches the area ofabsorbing layer 490, however, the refractive index of the absorbinglayer 490 is higher than that of the waveguide 420, which allows theexponentially decaying tail of the light outside the waveguide to beabsorbed in the absorbing layer. This is sometimes referred to asevanescent wave coupling. Without more, however, the amount of lightcoupled into the absorbing layer 490 per unit of contact area (contactbetween the waveguide and the absorbing layer) through this mechanismmay still be fairly low, requiring a relatively long strip of absorbinglayer (e.g., a millimeter) to absorb enough light to create a reliablephotodetector.

[0046] Grating 440 causes a portion of the light in the waveguide 420 tobe redirected towards the absorbing layer 490 at steeper angles, so thata larger percentage of the light travels into the absorbing layer 490 inthe region of the grating 440 than in the non-grating regions. Thus theabsorbing layer 490 may be smaller than in a conventional photodetectorbecause a greater percentage of light in the grating region is directedinto the absorbing layer. In one embodiment the absorbing layer 490 isapproximately 10 microns in length and width, but other embodiments mayhave absorbing layers with other dimensions. In a particular embodiment,the absorbing layer 490 is approximately the same in length and/or widthas the grating 440.

[0047] High Index Contrast Grating With Discontinuity

[0048]FIG. 5A shows a grating coupler with a discontinuity in thegrating structure to improve efficiency, according to one embodiment ofthe invention. The illustrated structure includes waveguide 520, grating540, and absorbing layer 590, as well as grating-side cladding 530, andcladding 510 on the opposite side of the waveguide in areas that are notcovered by absorbing layer 590. Although the illustrated embodimentshows the absorbing layer 590 connected directly to the waveguide 520,other embodiments may differ (e.g., one or more intermediate layers maybe disposed between the absorbing layer 590 and the waveguide 520).Unlike some of the couplers previously described, the refractive indexof the material in the grating 540 is higher than the refractive indexof the waveguide 520, and the refractive index of the waveguide 520 ishigher than the refractive index of the grating-side cladding 530.

[0049] In FIG. 5A, W is the width of each tooth in the grating, G is thewidth of the gap between adjacent teeth, and D_(G) is the width of thediscontinuity. In some embodiments, G is approximately equal to thewavelength of the intended light divided by 4n, and D_(G) isapproximately equal to the wavelength of the intended light divided by2n, where n is the refractive index of the material in the gap and inthe discontinuity (e.g., the grating-side cladding). In a particularembodiment, the intended wavelength is approximately 850 nm, G isapproximately 163 nm, DG is approximately 326 nm, W is approximately 105nm, and n is approximately 1.3, but other embodiments may use otherparameters.

[0050] The embodiment of FIG. 5A shows a discontinuity in the form of asingle extra-wide gap. FIG. 5B shows an alternate embodiment with adiscontinuity in the form of a single extra-wide tooth. In oneembodiment the extra-wide tooth has a width D_(T) of approximately onewavelength of the intended light divided by n, or double the width D_(G)of the gap discontinuity in FIG. 5A.

[0051] The efficiency of the grating, in terms of the amount of lightredirected per unit of grating area, is improved as the ratio of therefractive indices of the materials in the teeth and in the gaps isincreased. In one embodiment the material in the teeth comprises siliconnitride with a refractive index of approximately 2.0, and the materialin the gaps comprises silicon oxide with a refractive index ofapproximately 1.5, for a ratio of 4/3. A parameter defined as the indexcontrast is equal to

[0052] (n_(t)−n_(g))/n_(t), where n_(t) is the refractive index of thematerial in the teeth, and n_(g) is the refractive index of the materialin the gaps. In the above example, the index contrast would be(2.0−1.5)/2.0=1/4.

[0053] In one embodiment, the efficiency provided by the discontinuityis sufficiently great that the grating has no more than ten teeth, farfewer than with conventional gratings, with a correspondingly smallsize. Other embodiments may use other quantities of teeth. In theillustrated embodiments, the quantity of teeth on either side of thediscontinuity is the same, but other embodiments may have unequalquantities of teeth on either side of the discontinuity.

[0054]FIG. 6 shows a graph of optical coupling using a grating couplerwith a discontinuity and a high index contrast between the teeth and theadjacent cladding, according to one embodiment of the invention. In theillustrated embodiment, the vertical axis of the graph shows variouswavelengths of light that might be redirected by the grating coupler ofFIG. 5A. The horizontal axis shows the coupling, or redirection, ofthose wavelengths by the grating. As can be seen, light with awavelength of 1100 nm or longer has a coupling of 0.0, corresponding tono redirection at all (i.e., the light is not redirected by the gratingand continues to travel transversely through the waveguide). Light witha wavelength of 850 nm has a coupling of 1.0, corresponding to beingredirected at an angle of 45 degrees to the nominal direction of travelthrough the waveguide, which should be sufficient to redirect the lightout of the waveguide and into a sensor, absorbing layer, etc. aspreviously described. The effect of the grating/discontinuity on otherwavelengths may be read from the graph. Thus the size of thediscontinuity may be controlled to effectively redirect light of aparticular wavelength or band of wavelengths.

[0055] The foregoing description is intended to be illustrative and notlimiting. Variations will occur to those of skill in the art. Thosevariations are intended to be included in the various embodiments of theinvention, which are limited only by the spirit and scope of theappended claims.

1-6. (Canceled)
 7. A method, comprising: forming an optical sensor on afirst layer of material; forming an interlayer dielectric on the firstlayer of material and on the optical sensor; depositing a firstoptically transmissive material on the interlayer dielectric; depositinga second optically transmissive material on the first opticallytransmissive material to form a waveguide; forming an optical grating onthe second optically transmissive material; and depositing a thirdoptically transmissive material on the second optically transmissivematerial and on the grating.
 8. The method of claim 7, wherein: arefractive index of the first optically transmissive material is lessthan a refractive index of the second optically transmissvie material;and a refractive index of the third optically transmissive material isless than the refractive index of the first optically transmissivematerial.
 9. The method of claim 7, wherein: said forming the interlayerdielectric comprises forming electrical connections to the opticalsensor.
 10. The method of claim 7, wherein: said forming the interlayerdielectric comprises forming the interlayer dielectric with a thicknessof less than approximately 12 microns. 11-20. (Canceled)